In general, a PLL is a circuit designed to minimize a difference in frequency and/or phase between two signals. PLL circuits are widely used in applications where it is necessary to have two signals that have a known relationship to one another. For example, when transmitting information from a sending device to a receiving device, it is necessary to have a local clock of the receiving device in sync with a clock of the sending device so that the information can be reliably transmitted.
A PLL circuit can be implemented using various types of known architectures in which a forward control path of the PLL circuit includes separate proportional and integrating control paths. In a full digital implementation, both the proportional and integrating control paths are implemented using all digital components. An all-digital approach has advantages of being portable and scalable for other processes and applications. For example, an all-digital PLL implementation is needed for complex circuits such as memory devices, wherein a system clock of certain types of memory devices must be in sync with, for example, data so that data may be reliably written to or read from the memory. While an all-digital PLL architecture requires no analog circuitry, the digital nature of the phase measurement scheme can result in spurs in the output spectrum. Furthermore, the small signal parameters of an all-digital PLL can be a highly sensitive to deterministic jitter, resulting in changes in the PLL transfer function in the presence of noise on the power supplies.
In other known dual-path PLL architectures, a PLL circuit may be a “hybrid” framework in which both analog and digital components are employed in the control paths. For instance, hybrid PLL architecture may include a digital integrating control path and an analog proportional control path. Various circuit architectures and methods for implementing analog proportional control paths are known in the art, but these techniques have disadvantages of requiring significant amounts of complex analog circuitry. For example, in some known architectures, multiple operational amplifiers are required, in addition to a complex time-to-digital converter (TDC), or complex resettable switched resistor filters.